The invention relates to automatic needle insertion mechanisms for pen shaped syringes comprising a cartridge holder wherein a cartridge with a drug can be accommodated, and a dose setting and injection part by which a wanted dose may be set an subsequently pressed out from the cartridge by pressing a button projecting from the dose setting and injection part. As it will be understood the dose setting and injection part forms the more mechanically complicated part of the syringe whereas the cartridge holder is a rather simple tubular part. The designation xe2x80x9cpen shapedxe2x80x9d shall be taken broadly recognising that a pen need not be oblong and have a circular cross section. Syringes designed to apportion a number of set doses from an ampoule and commonly carried by the user are often called pens even when they do not have a shape making them look like a fountain pen as had the first syringes of that kind.
Pen syringes are mainly used by patients to perform self injections. By such handling by non professionals it is important that the handling is simple so that complications are avoided. A phase of the injection upon which most people look at with disinclination is the needle insertion phase. This disinclination may incur that the needle insertion is not performed as purposefully quickly as it is appropriate to obtain a precise, uncomplicated and painless insertion.
Consequently different apparatus for automatic needle insertion has been proposed. U.S. Pat. No. 4,787,891 describes an injection apparatus for common single dose syringes. In this apparatus the syringe is mounted in a holder which against the force of a spring may be drawn into a barrel and locked in its retracted position. When the barrel is placed where the. needle insertion is wanted and the lock is released, the holder with the syringe is forced forwards by the spring and a needle mounted on the syringe will project from the end of the barrel and pierce the skin where the barrel is placed.
From WO 88/08725 a pen shaped syringe is known wherein an automatic needle insert mechanism is integrated. The spring which provides the force necessary to insert the needle is further used to inject a set dose of medicine from the ampoule in the pen syringe. However, users may prefer to perform the injection themselves as this offers a possibility of adjusting the injection speed in accordance with the receiving ability of the tissue so that tensions and infiltrations are avoided.
Also pen shaped syringes with integrated needle insertion mechanisms but without automatic injection are known. Consequently, it is possible to buy pen syringes with built in needle insertion mechanisms.
If one has a pen syringe without such a mechanism, it will be attractive to have a separate needle insertion mechanism in which a pen shaped syringe may be mounted. This will give the user a freedom to the extent he may want to sometimes use a pen with and sometimes a pen without an automatic needle insertion mechanism.
The object of the invention is to provide such an automatic needle insertion mechanism for a syringes by which more individually set doses may be apportioned from a cylinder ampoule which can be changed when empty.
This is obtained by a pen shaped syringe of the kind described in the opening of this application, which syringe is according to the invention characterised in that the device comprises a mainly tubular housing in which a tubular pen holder in which a pen can be mounted can be axially displaced in a proximal direction to cock a spring which can thereafter be released to drive the pen holder with the pen a set distance in a distal direction. It shall be noticed that the designation xe2x80x9ctubularxe2x80x9d does not necessarily involve a tube having a circular cross section, the tube may have any appropriate cross section, e. g. a rectangular cross section.
In an appropriate embodiment of the invention the pen holder is shaped as a cartridge holder and only the dose setting and injection part of the pen syringe is mounted in the pen holder.
The cartridge holder forms a simple and inexpensive part of the pen syringe which part is designed to be in a simple way dismounted from the syringe when a new cartridge is going to be mounted. A cartridge holder with an automatic needle insertion mechanism can be seen as a spare part which can replace the common cartridge holder. If later a syringe without auto insertion device is wanted, the cartridge holder may again be replaced by the original simple one.
In a further appropriate embodiment of the mechanism according to the invention the cartridge holder shaped pen holder may at a proximal end have an inner thread in which an outer thread of the dose setting and injection part of a pen may be received.
According to the invention the pen holder may be provided with inspection windows to allow the same inspections as do a common cartridge holder. This way confusion of a user who is familiar with the pen with the original cartridge holder is avoided. It is an advantage that the means for operating the needle insertion device are firmly connected with the pen holder so that no superfluous operating means are left on the pen syringe when the pen holder with the automatic needle insertion device is replaced by the original cartridge holder.
According to an embodiment of the invention the distal end of the tubular housing of the automatic needle insertion device may be obliquely cut to allow abutment against the skin at an angle so that the needle may be inserted at an angle of about 45xc2x0.